Nitrogen Isotopes in Amino Acids and Nitrate Reveal Particle Production and Transformation in a Marine Oxygen-Deficient Zone

The eastern tropical North Pacific oxygen deficient zone (ETNP-ODZ) hosts unique assemblages of phytoplankton, microbes, and zooplankton, suggesting distinct particle production and transformation processes. This study compares δ¹⁵N values of nitrate in seawater and amino acids (AAs), such as phenylalanine (Phe) and glutamic acid (Glu), in size-fractionated particles collected across a productivity gradient within the ETNP-ODZ. Results show that at the primary chlorophyll maximum (PCM), phytoplankton relied more on NO₃-, while at sites with a pronounced secondary chlorophyll maximum (SCM), lower δ¹⁵N-Phe and δ¹⁵N-Glu values indicate a contribution from primary producers utilizing recycled nitrogen. Reduced ¹⁵N enrichment in Phe of small particles, along with smaller differences in δ¹⁵N-Phe between large and small particles compared to oxic waters, suggests slower microbial degradation of small particles in the ODZ. At the lower oxycline, distinctive δ¹⁵N-Phe and δ¹⁵N-Glu signatures were observed, possibly related to chemoautotrophic production and zooplankton interactions. These findings underscore the need for further research on nitrogen cycling at the SCM, particle alteration by zooplankton, and chemoautotroph contributions to deep-sea particles in expanding ODZs under climate change.

Presentation Preference: Oral

Primary Presenter: Lin Zhang, Texas A and M University Corpus Christi (lin.zhang@tamucc.edu)

Authors:

Wingman (Charlotte) Lee, Texas A and M University Corpus Christi  (wlee4@islander.tamucc.edu)

Mark Altabet, University of Massachusetts Dartmouth  (maltabet@umassd.edu)

Alanna Mnich, University of Massachusetts Dartmouth  (amnich@umassd.edu)

Lin Zhang, Texas A and M University Corpus Christi  (lin.zhang@tamucc.edu)